In computing systems, such as desktop computers, portable or laptop computers, servers, and others, storage devices are used to store data and program instructions. A disk-based storage device is one type of storage device; disk-based storage device include magnetic disk drives (e.g., a floppy disk drive or hard disk drive) and optical disk drives (e.g., a CD or DVD drive). Disk-based storage devices have a relatively large storage capacity. However, disk-based storage devices offer slower read-write speeds when compared to operating speeds of other components of a computing system, such as microprocessors and other semiconductor devices. A solid state memory device is another type of storage device; solid state memory devices include dynamic random access memory (DRAM), static random access memory (SRAM), flash memory, and electrically erasable and programmable read-only memory (EEPROM). Although solid state memory devices offer relatively high read-write speeds, usually on the order of nanoseconds, they have relatively limited storage capacities.
With improvements in nanotechnology, other types of storage devices are being developed. One such storage device is based on atomic force microscopy (AFM), in which one or more microscopic scanning probes are used to read and write to a storage medium. Storage of data in the storage medium is based on perturbations (dents) created by a tip of the probe in the surface of the storage medium. In one implementation, a dent represents a data bit “1, ” and the lack of a dent represents a data bit “0.” Other types of perturbations in the storage medium surface that can be used to convey data include creating or altering the topographic features or composition of the storage medium, altering the crystalline phase of the medium, filling or emptying existing electronic states of the medium, creating or altering domain structures or polarization states in the medium, creating or altering chemical bonds in the medium, employing tunneling effects to move and/or remove atoms or charge to or from the medium, or storing/removing charge from a particular region.
When the probe tip encounters and enters a dent, the tip (usually about 400° C.) transfers heat to the storage medium, which causes the temperature of the probe tip to fall, which in turn causes the electrical resistance of the tip to decrease. This decrease in resistance, which is a relatively tiny amount, is measured by detection circuitry that determines the state of the data bit. Another technique for detecting the state of a data bit uses a piezoresistive element in the probe. When the probe tip encounters a dent, the cantilever of the probe deflects, which causes the resistance of the piezoresistive element to change. This change in resistance is measured by detection circuitry.
However, reliable detection of data bits may not always be possible by the above techniques due to the relatively small change in resistance and the presence of noise and other factors.
To minimize friction and wear, it is desired to operate a probe with the minimum contact force required for proper operation. Due to manufacturing variations in the probe and other assembly tolerances creating variation in the amount of preloaded deflection in the probe, the nominal contact force may be significantly greater than the minimum required value.
Preventing the probe tip from catastrophically contacting the sample or media, e.g., in the case of a shock event, is important. A shock event not only has the potential to damage the local media and the corresponding data, but also to damage the tip itself, leading to the loss of data for the entire media area served by that tip.